Beginning at least as early as the 1950's, titanium was recognized to have properties making it attractive for use as structural armor against small arms projectiles. Investigation of titanium alloys for the same purpose followed. One titanium alloy known for use as ballistic armor is the Ti-6Al-4V alloy, which nominally comprises titanium, 6 weight percent aluminum, 4 weight percent vanadium and, typically, less than 0.20 weight percent oxygen. Another titanium alloy used in ballistic armor applications includes 6.0 weight percent aluminum, 2.0 weight percent iron, a relatively low oxygen content of 0.18 weight percent, less than 0.1 weight percent vanadium, and possibly other trace elements. Yet another titanium alloy that has been shown suitable for ballistic armor applications is the alpha-beta (α-β) titanium alloy of U.S. Pat. No. 5,980,655, issued Nov. 9, 1999 to Kosaka. In addition to titanium, the alloy claimed in the '655 patent, which is referred to herein as the “Kosaka alloy”, includes, in weight percentages, about 2.9 to about 5.0 aluminum, about 2.0 to about 3.0 vanadium, about 0.4 to about 2.0 iron, greater than 0.2 to about 0.3 oxygen, about 0.005 to about 0.03 carbon, about 0.001 to about 0.02 nitrogen, and less than about 0.5 of other elements.
Armor plates formed from the above titanium alloys have been shown to satisfy certain V50 standards established by the military to denote ballistic performance. These standards include those in, for example, MIL-DTL-96077F, “Detail Specification, Armor Plate, Titanium Alloy, Weldable”. The V50 is the average velocity of a specified projectile type that is required to penetrate an alloy plate having specified dimensions and positioned relative to the projectile firing point in a specified manner.
The above titanium alloys have been used to produce ballistic armor because when evaluated against many projectile types the titanium alloys provide better ballistic performance using less mass than steel or aluminum. Despite the fact that certain titanium alloys are more “mass efficient” than steel and aluminum against certain ballistic threats, there is a significant advantage to further improving the ballistic performance of known titanium alloys. Moreover, the process for producing ballistic armor plate from the above titanium alloys can be involved and expensive. For example, the '655 patent describes a method wherein a Kosaka alloy that has been thermomechanically processed by multiple forging steps to a mixed α-β microstructure is hot rolled and annealed to produce ballistic armor plate of a desired gauge. The surface of the hot rolled plate develops scale and oxides at the high processing temperatures, and must be conditioned by one or more surface treatment steps such as grinding, machining, shotblasting, pickling, etc. This complicates the fabrication process, results in yield losses, and increases the cost of the finished ballistic plate.
Given the advantageous strength-to-weight properties of certain titanium alloys used in ballistic armor applications, it would be desirable to fabricate articles other than ballistic plate from these alloys. However, it is generally believed that it is not possible to readily apply fabrication techniques other than simple hot rolling to many of these high-strength titanium alloys. For example, Ti-6Al-4V in plate form is considered too high in strength for cold rolling. Thus, the alloy is typically produced in sheet form via a complicated “pack rolling” process wherein two or more plates of Ti-6Al-4V having an intermediate thickness are stacked and enclosed in a steel can. The can and its contents are hot rolled, and the individual plates are then removed and ground, pickled and trimmed. The process is expensive and may have a low yield given the necessity to grind and pickle the surfaces of the individual sheets. Similarly, it is conventionally believed that the Kosaka alloy has relatively high resistance to flow at temperatures below the α-β rolling temperature range. Thus, it is not known to form articles other than ballistic plate from the Kosaka alloy, and it is only known to form such plate using the hot rolling technique generally described in the '655 patent. Hot rolling is suited to production of only relatively rudimentary product forms, and also requires relatively high energy input.
Considering the foregoing description of conventional methods of processing certain titanium alloys known for use in ballistic armor applications, there is a need for a method of processing such alloys to desired forms, including forms other than plate, without the expense, complexity, yield loss and energy input requirements of the known high temperature working processes.